Put the potatoes in a pan of salted water, bring to the boil and cook for 10-12 mins until a sharp knife can slide in easily. Drain and leave to steam-dry in the colander for 5 mins. Add the butter to the pan along with the milk and celery salt, if using, as well as a good crack of black pepper. Gently heat until the butter has melted, then return the potatoes to the pan and mash well. Stir in the soured cream and most of the herbs, and season to taste.

Meanwhile, heat the oil in a large frying pan over medium heat and cook the onions and mushrooms for 10-12 mins until turning golden at the edges. Mix in the paprika, marsala, leftover ragu and a splash of water. Bring to a simmer and cook for 5-7 mins until piping hot. Once cool, will keep frozen for up to three months. Reheat until piping hot. Stir in the mustard and double cream, then season to taste. Serve spooned over the herby mash with the remaining herbs scattered over.